1. Field of the Invention
The present invention relates to a method of electropolishing aided by ultrasonic means having the capability of rapidly discharging dregs. More specifically, the invention relates to a method of electrochemical polishing aided by an auxiliary ultrasonic apparatus which emits ultrasonic vibrating energy to effectively discharge dregs, shorten the polishing cycle time for each workpiece, and improve the surface roughness of the workpiece.
2. Description of Related Art
The current electropolishing process was ushered in at the dawn of modern electronic technology by Michael Faraday of the 18.sup.th century, who performed a series of brilliant experiments on the fundamental properties of electricity. The electrochemical polishing process, or electropolishing, utilizes a combination of electrical and chemical energy that would systematically remove a thin layer of surface material from a workpiece in order to polish the workpiece surface to a certain required surface roughness Ra. During an electropolishing process, a metallic workpiece is immersed in an electrolyte, which fills in the gap between the workpiece and the tool electrode. The tool electrode is connected with a cathode of a DC power source while the workpiece is connected with an anode of the source. In general, the rate of electrochemical-erosion, induced by the combined electrical and chemical energy, is basically determined by accessibility of the electrolyte to various topographical features of a workpiece surface. For example, a fairly rough surface area of a metal workpiece would constitute some micro-peaks where ions have easier access and some micro-troughs and micro-pores on the surface where less reaction can take place due to difficulty in access. Such electrochemical polishing, or electropolishing, method can usually overcome the difficulties in processing stainless steel or any other hardened steel materials of extreme hardness by conventional methods. Further, the electropolishing process is applicable to metal workpieces requiring further polishing after conventional or non-conventional machining steps, such as those of milling and electro-erosion, respectively. Specifically, the electropolishing process can replace conventional manual and mechanical polishing and deburring methods, where results are usually restricted by the skill and experience of the operating personnel; moreover, the cost of manual operation is almost always higher than an operation that can be automated by mechanical devices. Furthermore, the uneven contacting surface pressure of either an manual or a mechanical polishing method often causes under- or over-polishing of the workpiece, and a residual stress is hence created locally on the surface of the workpiece. Such residual stress sometimes exceeds the limit which the surface strength of subject steel workpiece is capable of withstanding and results in a surface collapsing. Large numbers of surface micro-pores are then scattered all over the surface causing a shortened expected operating life of a finished workpiece. Besides, training of qualified operating personnel in manual polishing is proving to be more and more difficult; further, mechanical polishing is sometimes limited by the geometrical shape and material characteristics of the workpieces. Therefore, electrochemical polishing, or electropolishing, can indeed improve drawbacks of polishing technique in the industry and can elevate the quality of traditional polishing.
Currently, electropolishing process has been applied on, for the most part, surface polishing of stainless steel materials. The electrochemical polishing process is usually performed on workpieces right after conventional or non-conventional machining steps such as those of milling and electro-erosion for opening up concavities, ridding surface reaction byproducts, providing erosion-resistance on processed surface area, and polishing the workpiece to a required surface roughness. The result is a chemical removal over the entire surface, but more accentuated in the ridges than in the troughs and resulting in a substantially smoother surface, and eliminating the micro-tearing and grooving which results from mechanical polishing techniques. Nevertheless, the conventional electropolishing requires the workpieces or metal parts to be completely secured and immersed in an electrolyte solution for an extended period of time for polishing, during which the workpiece is usually not actively cleaned and polished by any tool electrode or third-party polishing aid. The drawbacks of conventional electropolishing process are then lacking of active polishing aid, long polishing cycle time, and minimum material removal rate, whereby the conventional electropolishing process is basically limited to application in stainless steel polishing.
An alternative method for the conventional electropolishing method is an electropolishing using pulsing DC source to accomplish a rapid improvement of the surface roughness of the workpiece. Pulsing DC source employs a lower average electric current density without having to operate with high electrode flow rate as it is required by the conventional electropolishing method. This is because the heat and reaction byproducts are purged at each off-peak interval of the electric pulse. However, the non-conventional DC pulsing electropolishing method is limited in application in that the required polishing time is extended due to off-peak intervals during the pulsing process and in that the material removal rate is relatively small.
Furthermore, still another alternative to conventional electropolishing is an ultrasonic-aided machining process introduced since 1927, which has been playing an important role in the machining industry since. Currently, ultrasonic machining is applied in the fields of material science, mechanical and electric engineering's, chemistry, and even medical science. Ultrasonic energy is a high-frequency vibrating electric or magnetic energy generated by an ultrasonic oscillator, which in turn transforms a high-frequency electric energy into a high-frequency mechanic energy by means of electric or magnetic field change. Moreover, ultrasonic machining, or USM, operates in a liquid mixture of water and abrasive medium, or slurry, wherein an oscillation amplitude of 10.about.15 .mu.m and a high frequency of 15.about.30 kHz are preferably selected in order to rapidly clean and fine-polish the surface of the workpiece by means of a vibrating slurry.